IMAP is a mail server technology that, in short, allows users to store their messages on the remote server, so that they are accessible from any computer. Other email clients like Mozilla Thunderbird make it relatively simple to manage IMAP messages and folders, so I was surprised to hear some co-workers claim that Apple Mail doesn’t support this feature. After a bit of digging, I found that it does indeed; it’s just not obvious.
Thunderbird and other email clients refer to IMAP mail directories as “folders,” which makes sense to most folks familiar with file and operating systems. As it happens, Apple Mail (at least, Mail v3.1 in Leopard) calls these folders “Mailboxes,” which seems counterintuitive to most folks, who think of their email “mailboxes” as somewhat synonymous with their actual accounts.
At any rate, if you’re connecting to an IMAP server, the list of “Mailboxes” on the left-hand side of Apple Mail, below the name of your email account, is an actual list of your mail directories on the server. You can right-click (or Control+click) any of these Mailboxes to rename or delete your mail directories. Note: use this with care, as once you delete stuff on your IMAP server, it is gone for good!
You can also add a new mail folder by clicking the plus (+) icon in the bottom-left corner of Apple Mail, and choosing New Mailbox.
You can then enter a name for the folder, which will be added to the left-hand Mailboxes list, and the new folder will be created on your IMAP server. You can then store messages in your new folder, and those messages will be available to you anytime, anywhere, from any IMAP email client on any machine.